1. Field of the Invention
The present invention relates to a method for dissolving oxidized or non-oxidized radioactively contaminated surfaces from metal articles.
Articles of lead or lead-containing alloys are used in nuclear workplaces for shielding against radioactive radiation. It is known that a lead plate of an approximate thickness of 5 cm reduces radioactive radiation by a factor of 10. For this reason, shielding blocks are made of lead or lead alloys which are used to build entire walls around highly radioactive components. Pipes emitting strong radioactive radiation are shielded with lead mats. It is of course possible for these shielding blocks, lead mats and lead plates to become radioactively contaminated. Therefore, they must be decontaminated from time to time. Up to now this has not been done in a satisfactory manner. The surfaces of the lead or the lead-containing articles were scraped off or brushed by hand, the scraped off, contaminated material decontaminated and the remaining articles, still slightly radioactive, were melted down. The result was unsatisfactory and additionally resulted in spread of the radioactivity. Although the reclaimed articles of lead or lead-containing alloys could be reused, they exhibited increased radioactivity from the start. A second variant consisted of providing the lead shielding blocks or plates with a plastic covering, which was replaced from time to time. The contaminated plastic covering was decontaminated each time. Both variants resulted in a relatively large amount of waste which had to be decontaminated.
Lead articles are used in various nuclear applications. For example, in nuclear armaments, where lead components are used as reflector shields, among other uses, it is necessary to renew these lead components from time to time in order to maintain the operational readiness of the nuclear arms and to decontaminate the lead waste.
The same problems appearing in connection with lead and lead alloys are relevant in connection with other metals. For example, in installations for manufacturing UF.sub.6 in the civilian and military sectors there are large amounts of radioactively contaminated nickel. Although the value of these metals is high, only the smallest amounts could be reclaimed for reuse. An installation for manufacturing UF.sub.6 contains approximately 1,000 to 10,000 tons (metric) of pure nickel. Also, heat exchangers and steam generating installations of pressurized water reactors contain large amounts of nickel based alloys, such as Inocel 600 with a Ni content of approximately 70%. Both Cu and Cu alloys are also employed in heat exchangers and condensers of nuclear installations.
2. Description of the Prior Art
A method for the decontamination of radioactively contaminated metallic materials is known from U.S. Pat. No. 4,828,759. The radioactively contaminated metallic articles are placed into a bath containing fluoboric acid, which may be electro-chemically regenerated and the metals recovered and the regenerated fluoboric acid returned to the process. This method has proven too time-consuming for the decontamination of articles of lead and lead-containing alloys and, furthermore, is only usable at higher temperatures and concentrations. Solubility of lead and other metals, such as Ni, Cu, Hg, Ag or steel is a very slow process at room temperature even in HBF.sub.4 acid and it additionally generates hydrogen while taking place.
A possibility for removing lead contamination from copper alloys and steel is described in the publication "Metal Finishing Guidebook and Directory", Vol. 78, No. 1a, January 1980, page 505. Fluoboric acid and 30% hydrogen peroxide is recommended there for cleaning. Accordingly, it is intended to take off a thin layer of lead in cleaning of this type, while the underlying layer of different metals should not be altered, if possible. However, in the beginning of the above mentioned publication the recommendation for the use of hydrogen peroxide is qualified, since destruction of the surface may result. However, decontamination of radioactively contaminated lead is based on completely solid lead and the depth of removal as great as necessary.